In an optical transmission system, a wavelength division multiplexing (WDM) communication is widely used to increase transmission capacity by multiplexing a plurality of signal lights of different wavelengths. In this optical transmission system, since the signal is transmitted using signal lights attained by modulating the light with a signal, transmission capacity can be increased by increasing the multiplexing degree of wavelengths used in the WDM communication, or by increasing the bit rate of modulation.
As an external modulator for modulating the light outputted from a light source with an input signal, and then outputting the modulated signal, a Mach-Zehnder optical modulator is known in which an optical waveguide is formed on a substrate having the electro-optical effect (for example, Japanese Published Unexamined Patent Application No. 2002-182172).
FIGS. 6(a) to 6(d) are structural diagrams and cross-sectional views of a Mach-Zehnder optical modulator formed on a substrate having the electro-optical effect.
The Mach-Zehnder modulators illustrated in FIG. 6(a) and 6(c) include an optical waveguide and electrodes that are formed on a substrate 10A having the electro-optical effect. The input light is modulated and outputted due to mutual operation between the light propagated through the optical waveguide and the electric field generated by the electrodes. For the substrate 10A, lithium niobate (LiNb03; LN) and LiTa03 are used. However, since the maximum electro-optical effect can be attained when both the electric field and the polarizing direction of light are in the direction Z, the locations of the signal electrodes and optical waveguide may be different depending on the cutting direction of the substrate.
FIG. 6(a) illustrates a Mach-Zehnder optical modulator using the LN substrate 10A of X-cut, while FIG. 6(b) is a cross-sectional view along the line a–a′, shown in FIG. 6(a).
Waveguides 30A–30B, Y-branching waveguides 32A–32B, and parallel waveguides 31A–31B are optical waveguides formed on the substrate 10A. These waveguides are formed, for example, by forming a Ti film and then thermally diffusing the film while the film is patterned into the shape of waveguide, or performing the proton exchange in benzoic acid after a mask material is patterned.
A signal electrode 21A, and ground electrodes 22A and 22B, are formed on a buffer layer 11A formed on the substrate 10A. Electrodes made out of any conductive material, for example, gold (Au), may be used. Some Mach-Zehnder optical modulator may use a LN substrate 10A of the X-cut to attain mutual operation between the light propagated through the parallel waveguides 31A and 31B and the electric field in the direction Z. In such embodiments, the signal electrodes 21A and ground electrodes 22A–22B are formed in the shape of sandwiching the parallel waveguides 31A and 31B.
The light inputted to the optical modulator 2A is propagated through the optical waveguide 30A and is branched to the parallel waveguides 31A and 31B with a Y branching waveguide 32A. The signal electrode 21A is formed as a traveling wave electrode terminated with a resistor 4. An electric signal up to several tens of GHz is impressed to this signal electrode 21A from a signal source 3.
With the electric signal impressed to the signal electrode 21A, an electric field, indicated by the arrow marks of dotted lines in FIG. 6(b), is generated between the signal electrode 21A and the ground electrodes 22A–22B. Since the parallel waveguides 31A and 31B are held by the signal electrode and ground electrodes, the electric field applied to the parallel waveguide 31A is reversed in the direction of the electric field applied on the parallel waveguide 31B, and the refractive indices of the parallel waveguides 31A and 31B are respectively changed as Δn, −Δn.
Accordingly, since the refractive indices of the parallel waveguides 31A and 31B are different than each other, a phase difference is also generated in the lights propagated through the parallel waveguides 31A and 31B. The lights multiplexed with the Y branching waveguide 32B is outputted from the optical waveguide 30B. Such output changes typically depend on phase difference. When the phase difference is equal to a value (=π×odd number), the optical output becomes zero because the lights are cancelled with each other. However, the output is at a maximum when the phase difference is equal to a value (=π×even number).
The examples described above may be applied to a Mach-Zehnder optical modulator formed on a X-cut substrate, and moreover they may also be applied to a similar modulator formed on the Y-cut.
On the other hand, FIG. 6(c) illustrates the Mach-Zehnder type optical modulator that utilizes the Z-cut LN substrate 10A, and FIG. 6(d) is a cross-sectional view along line b–b′ of FIG. 6(c).
Like FIG. 6(a), waveguides 30A and 30B, Y branching waveguides 32A and 32B, and parallel waveguides 31A and 31B illustrated in FIG. 6(c), are formed on the substrate 10A, while the signal electrode 21A and ground electrodes 22A and 22B are formed on the buffer layer 11A.
As illustrated in FIG. 6(d), since the direction Z of the electric field resulting in the maximum efficiency of the electro-optical effect is different from that of X-cut in the Mach-Zehnder optical modulator formed on the Z-cut substrate, the parallel waveguide 31A is formed under the signal electrode 21A and the parallel waveguide 31B, under the ground electrode 22B. The buffer layer 11A serves to prevent absorption of the light propagated through the parallel waveguides 31A and 31B by the signal electrode 21A and ground electrode 22B.